The present invention relates generally to the reproduction of recorded signals. More particularly, the present invention relates to a method of and system for compensating a recorded signal for deviations in frequency caused by wow and flutter during recording and reproduction of the signal.
In the recording and reproduction of signals by passing a recording medium past a transducer, such as magnetic head, the density of the recorded signal depends upon the speed of the recording medium, as well as the frequency of the input signal. On reproduction, by passing the recording medium over the transducer, the frequency of the output signal is thus determined by the speed of the recording medium relative to the transducer, as well as the original information signal frequencies. Information recorded and reproduced in this manner usually experiences frequency deviations from the original signal which are more or less tolerable depending upon the intended use of the reproduced signal.
Various causes of these deviations can produce either a constant difference in frequency between the recorded and reproduced signal, a continually changing difference between these frequencies, or, a more or less periodic cyclic variation in the differences between these frequencies. Any substantial constant difference between recorded and reproduced frequencies can usually be eliminated by adjusting the recording speed to the reproducing speed, as for example, by utilizing a synchronous motor or the like. However, this approach involves a substantial expenditure for such a motor and control system therefor.
Continual unidirectional changes in frequency are not usually encountered, except as transients during acceleration and deceleration of the recording medium relative to the transducers. Cyclic changes in frequency are conventionally divided into two more or less overlapping categories. Deviations in the range from 0 to 1 Hertz, for example, are commonly termed "wow," whereas deviations between 1 and 100 Hertz or more are termed "flutter." The effects of wow and flutter on the reproduced sound, for example, of recorded music are not so drastic that moderate amounts cannot be tolerated, although precision turntables are employed where high fidelity is a requirement.
On the other hand, wow and flutter are especially serious in the reproduction of carrier signals which are frequency modulated at low information signal frequencies. Such signals would normally be preferred over amplitude modulated carriers for many purposes, such as to record and reproduce transducer signals which repeat conditions that change relatively slowly, e.g., the output of a cardiac monitor, a temperature recorder, a position transducer. In such a system, the information to be transferred is imposed on a basic frequency signal (carrier) by decreasing or increasing the carrier frequency in accordance with the information intelligence. Amplitude information is determined by carrier frequency change and frequency information is determined by the rate at which the carrier frequency changes. The percentage of carrier frequency change is typically 10 percent or less of the basic carrier frequency. These low frequencies are, therefore, typically in the frequency range affected by wow and flutter and it is not generally practical to record and reproduce such signals because of the difficulty of making an economical recorder in which wow and flutter are so reduced as to avoid interference with the recorded information.
The reduction of wow and flutter is especially important in apparatus used to electronically reproduce signatures, such as the signature reproducer described in U.S. Pat. No. 3,733,612. In that system, handwriting coordinates are divided into two channels which record the two-dimensional movement of a pen or stylus in terms of frequency. The carrier frequency disclosed in this apparatus is 1,400 Hz and the coordinate information superposed on the carrier causes the carrier to vary .+-.90 Hz. Therefore, the position information of the pen or stylus is in a bandwidth of 180 Hz. It will be apparent, therefore, that even minor variations in motor speed can result in serious distortion of the frequency information and ultimately the signature to be reproduced. Although a number of prior art devices attempt to overcome the aforementioned variations in tape speed by accurately controlling the capstan and tape speeds, such types of speed control devices are delicate and expensive, which generally makes them undesirable for use in many recording devices and environments.
In apparatus for recording and reproducing handwriting, accurate intelligence transmission is particularly critical. In addition, such equipment is most often used in an office environment where the operator is oftentimes neither especially skilled nor fully aware of the limitations of the equipment. Therefore, the use of delicate speed control devices for reducing wow and flutter in signature reproducers is undesirable.
Various other wow and flutter compensation schemes have been proposed in which a reference signal is recorded with the information signal. Circuits employing variable delay lines have been suggested to eliminate the effects of wow and flutter in an information signal by comparison with the error found between a simultaneously recorded pilot signal and a reference signal. In theory, such apparatus may be capable of making the desired correction. In practice, however, the variable delay line would be difficult to adjust and maintain in adjustment so that it would be particularly difficult to establish useful standards of performance which could be maintained from unit-to-unit and from day-to-day in a given unit.
One approach to solving the problem of wow and flutter in recording devices is suggested in U.S. Pat. No. 3,803,630 (related to earlier U.S. Pat. No. 3,582,956). The apparatus of the -630 patent transmits positional intelligence via frequency modes, which, if compared with a reference signal, produces an error correction signal during reproduction to account for tape speed variations between recording and playback. An analog divider circuit is utilized which is theoretically capable of perfect compensation. However, the assumption is made in the patent that the wow and flutter component in all the channels is equal, an assumption which does not permit the proper cancelling of the recording and playback errors since wow and flutter varies with the frequency deviation of an FM signal about its carrier frequency.
Another approach used to reduce wow and flutter in a recording system is disclosed in U.S. Pat. No. 3,872,504 which employs the concept of simultaneously recording a reference channel and linearly subtracting the wow and flutter from the desired channel via a mixing scheme. This approach has not been found to be a totally satisfactory solution since the amount of cancellation of the wow and flutter depends upon the exact frequency being processed. That is, the wow and flutter reduction factor decreases rapidly as the frequency deviates from the center carrier frequency, or that frequency where the compensation is theoretically perfect.
Another approach for reducing wow and flutter is disclosed in U.S. Pat. No. 3,839,729 wherein a pilot frequency is recorded with the information signal. Upon reproduction, the information signal is modulated with a signal derived from the pilot frequency to produce a sideband in which frequency deviation components introduced in recording and reproduction are absent. However, this approach is complex and costly to implement.